Microcalorimetry is an appropriate analytic tool for clinical chemistry assays in which the quantity to be measured is (1) a substrate for an available enzyme used as an analytic reagent, (2) an enzyme for which purified substrate is available as an analytic reagent, or (3) an inhibitory substance for an enzymic reaction where both substrate and enzyme are supplied as the analytic reagents. Cost-benefit may favor calorimetry over spectral methods where the reactions of interest do not develop color or where spectral interferences are inherent in the biological tissue or fluid to be evaluated. The present proposal is (1) for the modification of an existing Calvert-type instrument to increase its potential service load (by repetitive feed) and to improve the precision of the titrating apparatus in the microliter sample range; (2) for the demonstration of efficacy in the practical assay of aldolase (serum), hyaluronic acid (urine) and uric acid (serum and urine) as part of an ongoing clinical chemistry service. Evaluation in terms of sensitivity (sample size limitation), precision, accuracy, and cost-benefit with reference to authenic samples and clinical material will be made upon the basis of data obtained. Assuming appropriate instrument modification for routine use elsewhere and other reasonable conventions, projections will be made for cost- benefit analysis which will indicate the feasibility of the methods for wide-spread use in clinical chemistry.